LSR3 analysis: effects of TAAR1 agonists in animal models of psychosis
Francesca Tinsdeall, Fiona Ramage, Virginia Chiocchia and Malcolm Macleod
31 January 2024
Results
- Flow of study selection and descriptives
- 1.1 Description of experiment types and general approach
- TAAR1 Agonists v Control
- 2.1 Outcome 1: Locomotor activity
- 2.1.1 Risks of bias
- 2.1.2 Reporting completeness
- 2.1.3 Meta-analysis
- 2.1.4 Subgroup analyses and meta-regression
- Sex
- Category of disease induction
- Route of intervention administration
- Prophylactic or therapeutic intervention
- Duration of treatment period
- The intervention administered
- The efficacy of the drug
- The selectivity of the drug
- Potency of intervention
- Dose of intervention
- Standardised Dose
- SyRCLE RoB assessment
- Alternative SyRCLE RoB assessment
- ARRIVE guideline reporting completeness
- Alternative evaluation of ARRIVE reporting guidelines performance
- Heterogeneity explained by covariates
- 2.1.5 Sensitivity Analyses
- Imputed rho values of 0.2 and 0.8
- NMD
- 2.1.6 Reporting bias/small-study effects
- 2.2 Outcome 2: Cognitive function
- 2.2.1 Risks of bias
- 2.2.2 Reporting completeness
- 2.2.3 Meta-analysis
- 2.2.4 Subgroup analyses and meta-regression
- Sex
- Category of disease induction
- Route of intervention administration
- Prophylactic or therapeutic intervention
- Duration of treatment period
- The intervention administered
- The efficacy of the drug
- The selectivity of the drug
- Potency of intervention
- Dose of intervention
- Standardised Dose
- SyRCLE RoB assessment
- Alternative SyRCLE RoB assessment
- ARRIVE guideline reporting completeness
- Alternative evaluation of ARRIVE reporting guidelines performance
- Heterogeneity explained by covariates
- 2.2.5 Sensitivity Analyses
- Imputed rho values of 0.2 and 0.8
- NMD
- 2.2.6 Reporting bias/small-study effects
- 2.1 Outcome 1: Locomotor activity
- TAAR1 Agonist v known antipsychotic drug
- 3.1 Outcome 1: Locomotor activity
- 3.2 Outcome 2: Cognitive function
- Co-treatment with TAAR1 agonist plus know antipsychotic drug v known
antipsychotic drug alone
- 4.1 Outcome 1: Locomotor activity
- 4.2 Outcome 2: Cognitive function
- Effect of TAAR1 agonists in TAAR1 receptor knockout animals
- 5.1 Outcome 1: Locomotor activity
- 5.2 Outcome 2: Cognitive function
- Proportion of animals not progressing to treatment and adverse effects of treatment
- Summary of the evidence
- Software used
1. Flow of study selection and descriptives
The flow of study selection is shown in Figure 1. Studies included were published between 2011 and 2023. Overall, this analysis includes 15 studies containing 216 comparisons.
Figure 1 - PRISMA flowchart
The table below gives a summary of the included studies, the model and species used, the intervention tested, and the outcome measured. N represents an aggregate of animals contributing to outcomes reported from control and treatment groups, and if the same control group has contributed to more than one experiment, it will be counted twice.
| Study | Model | Strain | Comparison | Outcome | N |
|---|---|---|---|---|---|
| BEGNI, 2021 | Pharmacological | Lister hooded (rat) | SEP-363856 v Vehicle | Cognition | 40 |
| “ | “ | “ | “ | Locomotor activity | 120 |
| CINQUE, 2018 | Genetic | Wistar (rat) | RO5203648 v Vehicle | Cognition | 32 |
| DEDIC, 2019 | Pharmacological | C57BL/6J (mouse) | SEP-363856 v Vehicle | Locomotor activity | 48 |
| “ | “ | “ | SEP-363856 v clozapine | Locomotor activity | 48 |
| “ | “ | Sprague-dawley (rat) | SEP-363856 v Vehicle | Social interaction | 48 |
| “ | “ | “ | SEP-363856 v clozapine | Social interaction | 48 |
| GALLEY, 2012 | Pharmacological | Wistar (rat) | RO5073012 v Vehicle | Locomotor activity | 48 |
| KOKKINOU, 2021 | Pharmacological | C57BL/6 (mouse) | SEP-363856 v Vehicle | Neurobiological outcome | 17 |
| KRASAVIN, 2022a | Genetic | Wistar (rat) | LK000764 v Vehicle | Locomotor activity | 108 |
| KRASAVIN, 2022a | Pharmacological | Wistar (rat) | LK000764 v Vehicle | Locomotor activity | 140 |
| KRASAVIN, 2022b | Genetic | Wistar (rat) | AP163 v Vehicle | Locomotor activity | 18 |
| LEO, 2018 | Genetic | Wistar (rat) | RO5203648 v Vehicle | Locomotor activity | 24 |
| LIANG, 2022 | Pharmacological | ICR (mouse) | SEP-363856 & olanzapine v olanzapine | Cognition | 48 |
| “ | “ | “ | “ | Locomotor activity | 16 |
| “ | “ | “ | SEP-363856 v Vehicle | Cognition | 192 |
| “ | “ | “ | “ | Locomotor activity | 96 |
| “ | “ | “ | SEP-363856 v olanzapine | Cognition | 48 |
| “ | “ | “ | “ | Locomotor activity | 16 |
| REVEL, 2011 | Genetic | C57BL/6J (mouse) | RO5166017 v Vehicle | Locomotor activity | 42 |
| “ | Pharmacological | C57BL/6 (mouse) | RO5166017 v Vehicle | Locomotor activity | 200 |
| “ | “ | “ | “ | Stereotypy | 128 |
| “ | “ | NMRI (mouse) | RO5166017 v Vehicle | Locomotor activity | 84 |
| REVEL, 2012a | Genetic | C57Bl/6Jx129Sv/J (mouse) | RO5203648 v Vehicle | Locomotor activity | 48 |
| “ | Pharmacological | C57BL/6J (mouse) | RO5203648 v Vehicle | Locomotor activity | 154 |
| “ | “ | Wistar (rat) | RO5203648 v Vehicle | Locomotor activity | 84 |
| REVEL, 2012b | Pharmacological | C57BL/6J (mouse) | RO5073012 v Vehicle | Locomotor activity | 42 |
| REVEL, 2013 | Pharmacological | C57BL/6J (mouse) | RO5256390 v Vehicle | Locomotor activity | 122 |
| “ | “ | “ | RO5256390 v olanzapine | Locomotor activity | 32 |
| “ | “ | “ | RO5263397 & risperidone v risperidone | Locomotor activity | 96 |
| “ | “ | “ | RO5263397 v Vehicle | Locomotor activity | 184 |
| “ | “ | “ | RO5263397 v olanzapine | Locomotor activity | 80 |
| “ | “ | “ | RO5263397 v risperidone | Locomotor activity | 96 |
| “ | “ | Long-evans (rat) | RO5256390 v Vehicle | Cognition | 48 |
| “ | “ | Not”stated (mouse) | RO5256390 v Vehicle | Locomotor activity | 80 |
| “ | “ | “ | RO5263397 v Vehicle | Locomotor activity | 128 |
| SAARINEN, 2022 | Pharmacological | Not stated (mouse) | SEP-363856 v Vehicle | Locomotor activity | 56 |
| “ | “ | “ | “ | Prepulse inhibition | 60 |
| WANG, 2023 | Pharmacological | C57BL/6J (mouse) | Compound 50A v Vehicle | Locomotor activity | 72 |
| “ | “ | “ | Compound 50B v Vehicle | Locomotor activity | 90 |
| “ | “ | “ | Compound 50B v aripiprazole | Locomotor activity | 16 |
| “ | “ | “ | Compound 50B v risperidone | Locomotor activity | 16 |
| “ | “ | “ | SEP-363856 v Vehicle | Locomotor activity | 18 |
| “ | “ | “ | SEP-363856 v aripiprazole | Locomotor activity | 16 |
| “ | “ | “ | SEP-363856 v risperidone | Locomotor activity | 16 |
| “ | “ | “ | SEP-363856 v risperidone | Locomotor activity | 16 |
References of included studies are located in the appendix. Included studies used 38 unique disease model induction procedures.
1.1 Description of experiment types and methodological approach
Within the literature we identified distinct categories of experiments and the data presented would allow several meta-analytical contrasts to be drawn:
TAAR1 agonist vs control. These were experiments investigating the effect of administering a TAAR1 agonist alone, reported in 156 experiments from 15 publications.
TAAR1 agonist vs ‘known’ antipsychotic drug. These were experiments investigating the effect of administering a TAAR1 agonist alongside a currently licensed anti-psychotic reported in 27 experiments from 4 publications.
Co-treatment with TAAR1 agonist plus know antipsychotic drug v known antipsychotic drug alone, reported in 10 experiments from 2 publications.
Effect of TAAR1 antagonism on the effect of TAAR1 agonist v control. These were experiments investigating whether any effect of TAAR1 agonism was unhibited by TAAR1 antagonism. In this iteration of the review, all experiments within this category used genetic approaches to TAAR1 antogonism (that is, they knocked out the gene for the TAAR1 receptor, so any observed drug effect could not be due to actions mediated through the TAAR1 receptor and therefore could not be considered specific drug effects mediated through the TAAR1 receptor.
Each experiment type will be analysed separately. This is because each experiment type uses different control conditions.
In these studies the:
Control group is a group of animals that is (1) subjected to a psychosis model induction paradigm and (2) administered a control treatment (vehicle) or no treatment
Intervention group is a group of animals that is (1) subjected to a psychosis model induction paradigm and (2) administered a TAAR1 agonist treatment
Sham group is a group of animals that is (1) not subjected to a psychosis model induction paradigm and (2) administered a control treatment (vehicle) or no treatment. These data are required to allow a ‘normalised mean difference’ (NMD) effect size to be calculated, given by
\[ \frac{(\text{$\bar{\mu}_C - \bar{\mu}_T$})} {(\text{$\bar{\mu}_C - \bar{\mu}_S$)}} \text{ x 100} \]
where \(\bar{\mu}_C\), \(\bar{\mu}_T\), \(\bar{\mu}_S\) are the mean reported scores in the control, treatment, and sham groups respectively.
Outcomes with ≥2 independent effect sizes were considered for meta-analysis. In this iteration of the review, this includes locomotor activity and cognition.
All analyses were conducted allowing for the following hierarchical levels in a random effects model, which accounts for features common to experimental contrasts such as a shared control group:
Level 1: Rodent strain - effect sizes measured across experiments using the same rodent strain
Level 2: Study - effect sizes measured from different experiments presented in the same publication
Level 3: Experiment - effect sizes measured in the same experiment within a study, where often a control group contributes to several effect sizes
The hierarchical grouping may therefore be considered thus: Strains of laboratory animals are included in several Studies, each of which can report one or more Experiments, and each Experiment is comprised of at least two Cohorts which are considered identical except for differing in the experimental manipulation (the Intervention) or not being exposed to the disease modelling procedures (a Sham cohort, these only being used to provide a baseline for outcome measures to allow Normalised Mean Difference meta-analysis). An Experiment can include several experimental contrasts, for instance where different doses of drugs are compared to the same control group.
For some experimental contrasts, more than one locomotor or cognitive outcome - for instance both horizontal and vertical climbing activity - was measured in the same cohort of animals. Alternatively, some publications used the same drug doses with the same outcome measures in different experiments. For these reasons, some of the forest plots may appear to include ‘duplicate’ Study - Drug - Dose combinations with different outcomes. For the later, these are accounted for in the heirarchical analysis, but for the former there were insufficient levels of the different locomotor or cognitive outcome measures to allow for hierachical analysis and so this was not performed.
2 TAAR1 Agonists v Control
15 studies (156 comparisons) investigated the effects of TAAR1 Agonist versus Control. The number of studies and individual effect sizes for each outcome were:
Locomotor activity*: 13 studies and 125 comparisons in 8 strains
Prepulse inhibition*: 1 studies and 3 comparisons in 1 strain
Cognitive function: 4 studies and 19 comparisons in 4 strains
Social interaction: 1 studies and 3 comparisons in 1 strain
Stereotypy: 1 studies and 5 comparisons in 1 strain
* These outcomes were identified in the study protocol as primary outcomes of interest.
Only one publication reported each of prepulse inhibition (a primary outcome), social interaction, and stereotypy, and so these outcomes are not analysed further.
2.1 Outcome 1: Locomotor Activity
2.1.1 Risks of bias
Figure 2.1.1 shows the risk of bias summary for studies investigating the effect of administering a TAAR1 agonist on locomotor activity in animals. The risk of bias assessment was performed using the SyRCLE’s RoB tool.
Figure 2.1.1 - Traffic light plot of the risk of bias for locomotor activity
2.1.2 Reporting completeness
Figure 2.1.2 shows the reporting completeness summary for studies investigating the effect of administering a TAAR1 agonist on locomotor activity in animals. The reporting completeness assessment was performed using the ARRIVE guidelines.
Figure 2.1.2 - Traffic light plot of the reporting completeness for locomotor activity
2.1.3 Meta-analysis
The effect of administering a TAAR1 agonist on locomotor activity in animals using SMD as the effect size is shown in Figure 2.1.3. The pooled estimate for SMD across all individual comparisons is displayed as a diamond shape at the bottom of the plot. Dotted lines indicate the prediction interval of the pooled estimate.
Figure 2.1.3 - Forest plot of locomotor activity for TAAR1 Agonist vs control
For TAAR1 Agonist v Control, TAAR1 interventions had a pooled effect on locomotor activity of SMD = 1.032 (95% CI: 0.751 to 1.313, with a prediction interval of 0.008 to 2.056).
125 experimental comparisons were reported in 39 experiments reported from 13 publications and involving 8 different animal strains. The between strain variance was 0.017, the between study variance 0.016 and the within-experiment variance 0.14.
2.1.4 Subgroup analyses and meta-regressions
For each outcome, the covariates of interest for subgroup analysesand meta-regressions were:
Sex
Method of disease induction
Route of intervention administration
Whether the intervention was prophylactic or therapeutic (i.e. administered before or after disease model induction)
Duration of treatment period
The intervention administered
The efficacy of the drug (i.e. whether the drug is a partial or full agonist)
The selectivity of the drug
Potency of the intervention
Dose of intervention
We also conducted subgroup analyses using (1) SyRCLE Risk of Bias and (2) ARRIVE reporting completeness assessment scores as covariates to evaluate their influence on effect size estimates. These were not specified in the study protocol, but evaluation of risk of bias is required for the Summary of Evidence table, and no studies were considered at low risk of bias or high reporting completeness to allow such a sensitivity analysis
The significance (p value) reported is that for a test of whether the moderators are significantly different one from another, rather than whether the effect is significantly different from 0. Note that in this iteration of the review, insufficient data was available to conduct subgroup analysis for the following variables, as only one subgroup level was present: duration of treatment period
Sex
Figure 2.1.4.1 displays the estimates for the pooled SMD’s when comparisons are stratified by sex of the animal. Whiskers indicate the 95% confidence interval of each estimate. The overall pooled SMD, not stratified by sex, is displayed as a diamond shape at the bottom of the plot.
Figure 2.1.4.1 - Subgroup analysis of TAAR1 agonist vs control on locomotor activity by sex
The p-value for the association between the sex of animal groups used and outcome reported was 0.671. The between-strain variance was 0, the between-study variance 0.052, and the within-experiment variance 0.125.
Category of disease induction
Figure 2.1.4.2 displays the estimates for the pooled SMD’s when comparisons are stratified by the category of disease induction. Whiskers indicate the 95% confidence interval of each estimate. The overall pooled SMD, not stratified by category of disease induction, is displayed as a diamond shape at the bottom of the plot.
Figure 2.1.4.2 - Subgroup analysis of TAAR1 agonist vs control on locomotor activity by category of disease induction
The p-value for the association between whether genetic or pharmacological models were used and outcome reported was 0.709. The between-strain variance was 0.027, the between-study variance 0.016, and the within-experiment variance 0.145.
Route of intervention administration
Figure 2.1.4.3 displays the estimates for the pooled SMD’s when comparisons are stratified by the route of intervention administration. Whiskers indicate the 95% confidence interval of each estimate. The overall pooled SMD, not stratified by route of intervention administration, is displayed as a diamond shape at the bottom of the plot.
Figure 2.1.4.3 - Subgroup analysis of TAAR1 agonist vs control on locomotor activity by route of intervention administration
The p-value for the association between the route of intervention administration and outcome reported was 0.608. The between-strain variance was 0.032, the between-study variance 0.002, and the within-experiment variance 0.15.
Prophylactic or therapeutic intervention
Figure 2.1.4.4 displays the estimates for the pooled SMD’s when comparisons are stratified by whether the intervention was administered prophylactically or therapeutically. Whiskers indicate the 95% confidence interval of each estimate. The overall pooled SMD, not stratified by whether the intervention was administered prophylactically or therapeutically, is displayed as a diamond shape at the bottom of the plot.
Figure 2.1.4.4 - Subgroup analysis of TAAR1 agonist vs control on locomotor activity by intervention type
The p-value for the association between whether the intervention was administered prophylactically or therapeutically and outcome reported was 0.608. The between-strain variance was 0.032, the between-study variance 0.002, and the within-experiment variance 0.15.
Duration of treatment period
In this iteration of the review, all relevant comparisons administered the TAAR1 agonist for < 1 week. Therefore, no subgroup analyses were conducted for this variable.
The intervention administered
Figure 2.1.4.6 displays the estimates for the pooled SMD’s when comparisons are stratified by the intervention administered. Whiskers indicate the 95% confidence interval of each estimate. The overall pooled SMD, not stratified by the intervention administered, is displayed as a diamond shape at the bottom of the plot.
Figure 2.1.4.6 - Subgroup analysis of TAAR1 agonist vs control on locomotor activity by intervention administered
The p-value for the association between the intervention and outcome reported was 0.448. The between-strain variance was 0.063, the between-study variance 0, and the within-experiment variance 0.117.
The efficacy of the drug (i.e. whether the drug is a partial or full agonist)
Figure 2.1.4.7 displays the estimates for the pooled SMD’s when comparisons are stratified by the action/efficacy of the intervention administered. Whiskers indicate the 95% confidence interval of each estimate. The overall pooled SMD, not stratified by intervention efficacy, is displayed as a diamond shape at the bottom of the plot.
Figure 2.1.4.7 - Subgroup analysis of TAAR1 agonist vs control on locomotor activity by efficacy of the drug
The p-value for the association between whether the drug was a full or partial agonist and outcome reported was 0.318. The between-strain variance was 0.024, the between-study variance 0.03, and the within-experiment variance 0.129.
The selectivity of the drug
Figure 2.1.4.8 displays the estimates for the pooled SMD’s when comparisons are stratified by the selectivity of the intervention administered. Whiskers indicate the 95% confidence interval of each estimate. The overall pooled SMD, not stratified by intervention selectivity, is displayed as a diamond shape at the bottom of the plot.
Figure 2.1.4.8 - Subgroup analysis of TAAR1 agonist vs control on locomotor activity by selectivity of the drug
The p-value for the association between whether the drug was highly selective, or also manifests 5-HT1A effects, and outcome reported was 0.297. The between-strain variance was 0, the between-study variance 0.016, and the within-experiment variance 0.146.
Potency of intervention
The pEC50 value of each drug was used to measure potency. The pEC50 value is the negative logarithm (to base 10) of the EC50 value. Higher pEC50 values indicate higher potency (as they indicate a lower EC50). Figure 2.1.4.9 displays a visualisation of the meta-regression using the pEC50 value as an explanatory variable. Dashed lines represent the 95% confidence interval of the regression line. The dotted lines represent the 95% prediction interval. Raw data are plotted with ‘bubble’ size adjusted according to effect size precision.
Figure 2.1.4.9 - Meta-regression of TAAR1 agonist vs control on locomotor activity by potency of intervention
The estimate for \(\beta\) was -0.021 (p = 0.907). The between-strain variance was 0, the between-study variance 0.041, and the within-experiment variance 0.142.
Dose of intervention
In this iteration of the review, the TAAR1 agonists tested against control for their effect on locomotor activity were: RO5203648, RO5263397, SEP-363856, RO5166017, LK000764, RO5256390, Compound 50B, Compound 50A, RO5073012 and AP163. The dashed lines in the plot represent the 95% confidence interval of the regression line and the dotted lines represent the 95% prediction interval. Raw data are plotted with point size adjusted according to effect size precision.
RO5203648: There were 21 comparisons from 2 publication(s).
RO5263397: There were 21 comparisons from 1 publication(s).
SEP-363856 (Ultaront): There were 19 comparisons from 5 publication(s).
RO5166017: There were 18 comparisons from 1 publication(s).
LK000764: There were 16 comparisons from 1 publication(s).
RO5256390: There were 14 comparisons from 1 publication(s).
Compound 50B: There were 5 comparisons from 1 publication(s).
Compound 50A: There were 4 comparisons from 1 publication(s).
RO5073012: There were 4 comparisons from 2 publication(s).
AP163: There were 3 comparisons from 1 publication(s).
Standardised dose
We then sought evidence of a dose response relationship across all drugs. To do this, we conducted meta-regression using a constructed variable, the ‘standardised dose’. The EC50 of a drug is the molar concentration at which 50% of the maximal response occurs. While the drug concentrations achieved at the receptor are unknown, we can approximate this from the dose given (expressed as g/kg), and the molar mass of the drug (g/mol). This relies on an approximation that the drug is equally distributed throughout the animal, and so does not take into account for example first pass metabolism for orally administered drugs, blood brain barrier solubility or differential accumulation in fatty tissues. As such, it should be interpreted with extreme caution; but does provide allow some imputation of whether, across all drugs, there is a dose-response effect. On this measure, a standardised dose of 0 would reflect 50% of maximum effect and a standardised dose of 1 would reflect around 80% of maximum effect
The standardised dose was calculated as the logarithm of the dose of the intervention (in g/kg) divided by the product of the intervention’s EC50 (in moles) and the Molar mass of the drug (in g/mol):
\[ \log\frac{(\text{Dose of Intervention (g/kg)})}{(\text{Molar Mass (g/mol)}) \times ({\text{EC50 (mol)}})} \]
This is a simplified approximation based on the reasoning that if drug actions are mediated through the TAAR1 receptor, and drug efficacy is reflected in the respective EC50 values, then in principal drugs should exhibit similar effects when acting at their respective EC50.
The actual concentration of a drug at the receptor site is influenced by several variables, including dosage, administration route, elimination half-life, and first-pass metabolism (in case of oral administration). Incorporating all these factors accurately would necessitate a detailed pharmacokinetic model, which falls outside the scope of this review. Consequently, we assume uniformity across experiments in terms of (i) volume of distribution, (ii) first-pass metabolism, (iii) blood-brain barrier permeability, and (iv) experimental design, especially regarding the timing of peak drug concentration (where we assume that experiments were designed to be done at a time when the drug was near peak concentration).
Figure 2.1.4.10 provides a visualisation of the meta-regression analysis relationship between standardised doses of TAAR1 agonists and the Standardized Mean Difference (SMD) change in Locomotor activity. As before, dashed lines represent the 95% confidence interval of the regression line and dotted lines represent the 95% prediction interval. Raw data are plotted with point size adjusted according to effect size precision.
Figure 2.1.4.10 - Meta regression of standardised dose for TAAR1 agonist vs control on locomotor activity
The estimate for the change in effect per log unit change in standardised dose was 0.165 (p = 125). The between-strain variance was 0, the between-study variance 0.438, and the within-experiment variance 0.095.
SyRCLE RoB assessment
Figure 2.1.4.11 displays the estimates for the pooled SMD’s when comparisons are stratified by how many of the SyRCLE risk of bias assessment criteria (of which there are 10) that the experiment met. Whiskers indicate the 95% confidence interval of each estimate. The overall pooled SMD, not stratified by SyRCLE Risk of Bias, is displayed as a diamond shape at the bottom of the plot.
Figure 2.1.4.11 - Subgroup analysis of TAAR1 agonist vs control on locomotor activity by SyRCLE RoB criteria met
The p-value for the association between SyRCLE Risks of Bias reporting and outcome reported was 0.215. The between-strain variance was 0.017, the between-study variance 0, and the within-experiment variance 0.141.
Alternative SyRCLE RoB assessment
Figure 2.1.4.12 displays the estimates for the pooled SMD’s when comparisons are stratified by whether of not any of the SyRCLE Risk of bias domains were rated as low risk of bias. Whiskers indicate the 95% confidence interval of each estimate. The overall pooled SMD, not stratified by SyRCLE Risk of Bias, is displayed as a diamond shape at the bottom of the plot.
Figure 2.1.4.12 - Subgroup analysis of TAAR1 agonist vs control on locomotor activity by alternative SyRCLE RoB assessment
The p-value for the association between low SyRCLE Risks of Bias reporting and outcome reported was 0.229. The between-strain variance was 0.001, the between-study variance 0, and the within-experiment variance 0.162.
ARRIVE reporting completeness guidelines
Experiments were categorised based on the number of ARRIVE guidelines items (of which there are 23) met.
Figure 2.1.4.13 displays a visualisation of the meta-regression using the number of ARRIVE items met as an explanatory variable. Dashed lines represent the 95% confidence interval of the regression line. The dotted lines represent the 95% prediction interval. Raw data are plotted with ‘bubble’ size adjusted according to effect size precision.
Figure 2.1.4.13 - Meta-regression of number of ARRIVE items met for TAAR1 agonist vs control on locomotor activity
The estimate for \(\beta\) was -0.004 (p = 0.923). The between-strain variance was 0, the between-study variance 0.041, and the within-experiment variance 0.141.
Heterogeneity explained by covariates (TAAR1 Agonist vs Control on locomotor activity)
The table below shows which of the covariates, if any, explain some of the heterogeneity observed in the effect sizes of the effect of TAAR1 agonists on locomotor activity. We present marginal R2, which measures the proportion of variance explained by including moderators in the model (the % change in the between-studies variance when the covariate is included in the model). The coefficients are derived form an RMA model fitted with an intercept (and so represent, for each category, the point estimate and 95% CIs of the effect in that category).
| Moderator | Category | \(\beta\) | 95% CI | Marginal R2 (%) |
|---|---|---|---|---|
| Overall effect | - | 1.032 | 0.751 - 1.313 | - |
| Sex | - | - | - | 8.6% |
| - | Female | 0.847 | 0.011 - 1.683 | - |
| - | Male | 1.185 | 0.818 - 1.551 | - |
| - | Mixed | 0.819 | 0.169 - 1.469 | - |
| - | Not reported | 0.964 | 0.604 - 1.323 | - |
| Category of disease model induction | - | - | - | 0.9% |
| - | Genetic | 1.139 | 0.516 - 1.891 | - |
| - | Pharmacological | 1.018 | 0.749 - 1.288 | - |
| Administration route | - | - | - | 2.8% |
| - | Intraperitoneal | 0.866 | 0.109 - 1.622 | - |
| - | Oral | 1.06 | 0.78 - 1.34 | - |
| Prophylactic or therapeutic intervention | - | - | - | 2.8% |
| - | Prophylactic | 1.06 | 0.78 - 1.34 | - |
| - | Therapeutic | 0.866 | 0.109 - 1.622 | - |
| Intervention administered | - | - | - | 41.6% |
| - | AP163 | 1.319 | -0.677 - 3.315 | - |
| - | Compound 50A | 0.665 | -0.459 - 1.79 | - |
| - | Compound 50B | 1.109 | -2.086 - 1.666 | - |
| - | LK000764 | 0.407 | -0.575 - 1.388 | - |
| - | RO5073012 | 0.6 | -0.293 - 1.492 | - |
| - | RO5166017 | 1.324 | 0.685 - 1.963 | - |
| - | RO5203648 | 1.006 | 0.406 - 1.607 | - |
| - | RO5256390 | 1.694 | 0.976 - 2.412 | - |
| - | RO5263397 | 0.929 | 0.339 - 1.518 | - |
| - | SEP-363856 (Ultaront) | 0.936 | 0.476 - 1.397 | - |
| Drug efficacy | - | - | - | 5.3% |
| - | Full agonist | 1.108 | 0.809 - 1.407 | - |
| - | Partial agonist | 0.906 | 0.561 - 1.25 | - |
| Drug selectivity | - | - | - | 18.7% |
| - | High | 1.14 | 0.864 - 1.417 | - |
| - | Low | 0.945 | 0.534 - 1.356 | - |
| - | Unclear | 0.637 | 0.014 - 1.261 | - |
| Drug potency | per log unit | -0.021 | -0.382 - 0.339 | 0.1% |
| Standardised drug dose | per log unit | 0.2 | 0.151 - 0.248 | 34.2% |
| Risk of Bias | - | - | - | 22.1% |
| - | 0 criteria met | 1.105 | 0.842 - 1.368 | - |
| - | 1 criteria met | 0.593 | -0.014 - 1.201 | - |
| - | 2 criteria met | 1.545 | 0.237 - 2.854 | - |
| Reporting completeness | - | - | - | 3.1% |
| - | 4-7 criteria met | 1.471 | -0.424 - 3.367 | - |
| - | 8-11 criteria met | 1.07 | 0.69 - 1.45 | - |
| - | 12-15 criteria met | 0.972 | 0.55 - 1.393 | - |
| - | 16-19 criteria met | 1.096 | -0.119 - 2.311 | - |
2.1.5 Sensitivity Analyses
We examine the robustness of the findings for the primary outcome by performing the following sensitivity analyses
Imputed 𝞺 values of 0.2 and 0.8
In the previous analyses for the effect of TAAR1 agonists on locomotor activity, we imputed a \(\rho\) value - the imputed within-study correlation between observed effect sizes - of 0.5. Here, we examine the effect of imputing \(\rho\) values of 0.2 and 0.8.
When the \(\rho\) value is assumed to be 0.2, the TAAR1 interventions had a larger effect on locomotor activity of SMD = 1.15 (95% CI: 0.91 to 1.4) with a prediction interval of 0.09 to 2.21).
When the \(\rho\) value is assumed to be 0.8, the TAAR1 interventions had a smaller and more imprecise effect on locomotor activity of SMD = 0.72 (95% CI: 0.07 to 1.38) with a prediction interval of -1.33 to 2.78).
For reference the pooled effect size when rho is assumed to be 0.5 is 1.03 (95% CI: 0.75 to 1.31). Therefore, the effect is very sensitive to imputed within-study correlation between effect sizes.
Normalised Mean Difference (NMD)
For locomotor activity, 96 out of 125 comparisons, i.e. 76.8 % of comparisons, had data available for a Sham group and, therefore, to calculate an NMD.
The effect of administering a TAAR1 agonist on locomotor activity in animals using NMD as the effect size is shown in Figure 2.1.5. The pooled estimate for NMD across all individual comparisons is displayed as a diamond shape at the bottom of the plot. Dotted lines indicate the prediction interval of the pooled estimate.
Figure 2.1.5 - Forest plot of TAAR1 agonist vs control on locomotor activity using NMD
For TAAR1 Agonist v Control, TAAR1 interventions had a pooled effect on locomotor activity of NMD = 56.86 (95% CI: 44.08 to 69.63) with a prediction interval of -13.72 to 127.44). For reference the pooled effect size for SMD was 1.03 (95% CI: 0.75 to 1.31).
96 experimental comparisons were reported in 30 experiments reported from 10 publications and involving 8 different animal strains. Between-strain variance was 0, between-study variance was 0.02, and within-study variance (between experiments) was 861.72.
Robust variance estimator (RVE)
Here, we examine the robustness of results when using a sandwich-type estimator to obtain cluster-robust tests and confidence intervals of the model coefficients. The variance-covariance matrix is estimated using the ‘bias-reduced linearization’ for small-sample adjustment and Strain as a clustering variable.
When using the robust variance estimator, TAAR1 interventions had a pooled effect on locomotor activity of SMD = 1.03 (95% CI: 0.73 to 1.33 with a prediction interval of -0.12 to 2.18). For reference the pooled effect size for SMD was 1.03 (95% CI: 0.75 to 1.31), so the using a robust variance estimator does not substantially change the results.
2.1.6 Reporting bias/small-study effects
Because of the relationship between SMD effect sizes and variance inherent in their calculation, where study size is small the standard approach to seeking evidence of small-study effects (regression based tests including Egger’s regression test for multilevel meta-analysis) can lead to over-estimation of small-study effect (see for instance 10.7554/eLife.24260). To address this we used Egger’s regression test for multilevel meta-analysis, with regression of SMD effect size against 1/√N, where N is the total number of animals involved in an experiment.
Egger regression based on 125 effects of TAAR1 Agonist v Control where Locomotor activity was measured showed a coefficient for small-study effect of 7.93 (95% CI: 3.05 to 12.82; p = 0.002).
2.2 Outcome 2: Cognitive function
2.2.1 Risks of bias
Figure 2.2.1 shows the risk of bias summary for studies investigating the effect of administering a TAAR1 agonist on cognition in animals. The risk of bias assessment was performed using the SyRCLE’s RoB tool.
Figure 2.2.1 - Traffic light plot of the risk of bias for cognitive function
2.2.2 Reporting completeness
Figure 2.2.2 shows the reporting completeness summary for studies investigating the effect of administering a TAAR1 agonist on cognition in animals. The reporting completeness assessment was performed using the ARRIVE guidelines.
Figure 2.2.2b - Traffic light plot of the reporting completeness for cognitive function
2.2.3 Meta-analysis
The effect of administering a TAAR1 agonist on cognitive outcomes in animals using SMD as the effect size is shown in Figure 2.2.3. The pooled estimate for SMD across all individual comparisons is displayed as a diamond shape at the bottom of the plot. Dotted lines indicate the prediction interval of the pooled estimate.
Figure 2.2.3 - Forest plot of cognitive function for TAAR1 Agonist vs control
For TAAR1 Agonist v Control, TAAR1 interventions had a pooled effect on cognitive outcomes of SMD =0.8 (95% CI: -0.301 to 1.9, with a prediction interval of-1.609 to 3.208).
19 experimental comparisons were reported in NA experiments reported from 4 publications and involving 5 different animal strains. The between strain variance was 0.6, the between study variance NA and the within-experiment variance NA.
2.2.4 Subgroup analyses and meta-regressions
Sex
Figure 2.2.4.1 displays the estimates for the pooled SMD’s when comparisons are stratified by sex of the animal. Whiskers indicate the 95% confidence interval of each estimate. The overall pooled SMD, not stratified by sex, is displayed as a diamond shape at the bottom of the plot.
Figure 2.2.4.1 - Subgroup analysis of TAAR1 agonist vs control on cognitive function by sex
The p-value for the association between the sex of animal groups used and outcome reported was 0.46. The between-strain variance was 0.24, the between-study variance 0.24, and the within-experiment variance 0.
Category of disease induction
Figure 2.2.4.2 displays the estimates for the pooled SMD’s when comparisons are stratified by the category of disease induction. Whiskers indicate the 95% confidence interval of each estimate. The overall pooled SMD, not stratified by category of disease induction, is displayed as a diamond shape at the bottom of the plot.
Figure 2.2.4.2 - Subgroup analysis of TAAR1 agonist vs control on cognitive function by category of disease induction
The p-value for the association between whether genetic or pharmacological models were used and outcome reported was 0.27. The between-strain variance was 0.27, the between-study variance 0.27, and the within-experiment variance 0.
Route of intervention administration
Figure 2.2.4.3 displays the estimates for the pooled SMD’s when comparisons are stratified by the administration route of the intervention. Whiskers indicate the 95% confidence interval of each estimate. The overall pooled SMD, not stratified by administration route of the intervention, is displayed as a diamond shape at the bottom of the plot.
Figure 2.2.4.3 - Subgroup analysis of TAAR1 agonist vs control on cognitive function by route of intervention administration
The p-value for the association between whether genetic or pharmacological models were used and outcome reported was 0.27. The between-strain variance was 0.27, the between-study variance 0.27, and the within-experiment variance 0.
Prophylactic or therapeutic intervention
In this iteration of the review, all relevant comparisons administered the TAAR1 agonist after induction of the disease model. Therefore, no subgroup analyses were conducted for this variable.
Duration of treatment period
In this iteration of the review, all relevant comparisons administered the TAAR1 agonist for < 1 week. Therefore, no subgroup analyses were conducted for this variable.
The intervention administered
Figure 2.2.4.4 displays the estimates for the pooled SMD’s when comparisons are stratified by the intervention administered. Whiskers indicate the 95% confidence interval of each estimate. The overall pooled SMD, not stratified by the intervention administered, is displayed as a diamond shape at the bottom of the plot.
Figure 2.2.4.4 - Subgroup analysis of TAAR1 agonist vs control on cognitive function by intervention administered
The p-value for the association between the intervention and outcome reported was 0.64. The between-strain variance was 0.58, the between-study variance 0.58, and the within-experiment variance 0.
The efficacy of the drug (i.e. whether the drug is a partial or full agonist)
In this iteration of the review, all relevant comparisons administered the TAAR1 agonists with partial agonist activity. Therefore, no subgroup analyses were conducted for this variable.
The selectivity of the drug
Figure 2.2.4.5 displays the estimates for the pooled SMD’s when comparisons are stratified by the selectivity of the intervention administered. Whiskers indicate the 95% confidence interval of each estimate. The overall pooled SMD, not stratified by intervention selectivity, is displayed as a diamond shape at the bottom of the plot.
Figure 2.2.4.5 - Subgroup analysis of TAAR1 agonist vs control on cognitive function by selectivity of the drug
The p-value for the association between whether the drug was highly selective, or also manifests 5-HT1A effects, was 0.38. The between-strain variance was 0.41, the between-study variance 0.41, and the within-experiment variance 0.
Potency of interventions
The pEC50 value of each drug was used to measure potency. The pEC50 value is the negative logarithm (to base 10) of the EC50 value. Higher pEC50 values indicate higher potency (as they indicate a lower EC50). Figure 2.2.4.6 displays a visualisation of the meta-regression using the pEC50 value as an explanatory variable. Dashed lines represent the 95% confidence interval of the regression line. The dotted lines represent the 95% prediction interval. Raw data are plotted with ‘bubble’ size adjusted according to effect size precision.
Figure 2.2.4.6 - Meta-regression of TAAR1 agonist vs control on cognitive function by potency of the interventions
The estimate for \(\beta\) was -1.19 (p = 0.47). The between-strain variance was 0.5, the between-study variance 0.5, and the within-experiment variance 0.
Dose of intervention
In this iteration of the review, the TAAR1 agonists tested against control for their effect on cognition were; SEP-363856, RO5256390 and RO5203648. The dashed lines in the plot represent the 95% confidence interval of the regression line and the dotted lines represent the 95% prediction interval. Raw data are plotted with point size adjusted according to effect size precision.
RO5203648: There were 2 comparisons from 1 publication(s).
RO5263397: There were 0 comparisons from 0 publication(s).
SEP-363856 (Ultaront): There were 14 comparisons from 2 publication(s).
RO5166017: There were 0 comparisons from 0 publication(s).
LK000764: There were 0 comparisons from 0 publication(s).
RO5256390: There were 3 comparisons from 1 publication(s).
Compound 50B: There were 0 comparisons from 0 publication(s).
Compound 50A: There were 0 comparisons from 0 publication(s).
RO5073012: There were 0 comparisons from 0 publication(s).
AP163: There were 0 comparisons from 0 publication(s).
Standardised dose
We then sought evidence of a dose response relationship across all drugs using the approach described for locomotor activity.
Figure 2.2.4.7 provides a visualisation of the meta-regression analysis relationship between standardised doses of TAAR1 agonists and the Standardized Mean Difference (SMD) change in cognition. As before, dashed lines represent the 95% confidence interval of the regression line and dotted lines represent the 95% prediction interval. Raw data are plotted with point size adjusted according to effect size precision.
Figure 2.2.4.7 - Meta regression of standardised dose for TAAR1 agonist vs control on cognitive function
The estimate for \(\theta\) was 1.88. The estimate for \(\beta\) was -0.21.
SyRCLE RoB assessment
Figure 2.2.4.8 displays the estimates for the pooled SMD’s when comparisons are stratified by how many of the SyRCLE risk of bias assessment criteria (of which there are 10) that the experiment met. Whiskers indicate the 95% confidence interval of each estimate. The overall pooled SMD, not stratified by SyRCLE RoB assessment, is displayed as a diamond shape at the bottom of the plot.
Figure 2.2.4.8 - Subgroup analysis of TAAR1 agonist vs control on cognitive function by SyRCLE RoB criteria met
The p-value for the association between SyRCLE Risks of Bias reporting and outcome reported was 0.13. The between-strain variance was 0.09, the between-study variance 0.09, and the within-experiment variance 0.
Alternative SyRCLE RoB assessment
Figure 2.2.4.9 displays the estimates for the pooled SMD’s when comparisons are stratified by whether of not any of the SyRCLE Risk of bias domains were rated as low risk of bias. Whiskers indicate the 95% confidence interval of each estimate. The overall pooled SMD, not stratified by SyRCLE Risk of Bias, is displayed as a diamond shape at the bottom of the plot.
Figure 2.2.4.9 - Subgroup analysis of TAAR1 agonist vs control on cognitive function by alternative SyRCLE RoB assessment
The p-value for the association between low SyRCLE Risks of Bias reporting and outcome reported was 0.13. The between-strain variance was 0.09, the between-study variance 0.09, and the within-experiment variance 0.
ARRIVE reporting completeness guidelines
Experiments were categorised based on the number of ARRIVE guidelines items (of which there are 23) met.
Figure 2.2.4.10 displays a visualisation of the meta-regression using the number of ARRIVE items met as an explanatory variable. Dashed lines represent the 95% confidence interval of the regression line. The dotted lines represent the 95% prediction interval. Raw data are plotted with ‘bubble’ size adjusted according to effect size precision.
Figure 2.2.4.10 - Meta-regression of number of ARRIVE items met for TAAR1 agonist vs control on cognitive function
The estimate for \(\beta\) was -0.01 (p = 0.97). The between-strain variance was 0.72, the between-study variance 0.72, and the within-experiment variance 0.
Heterogeneity explained by covariates (TAAR1 Agonist vs Control on cognitive function)
The table below shows which of the covariates, if any, explain some of the heterogeneity observed in the effect sizes of the effect of TAAR1 agonists on cognition. We present marginal R2, which measures the proportion of variance explained by including moderators in the model.
| Moderator | Category | \(\beta\) | 95% CI | Marginal R2 (%) |
|---|---|---|---|---|
| Overall effect | - | 0.8 | -0.3 - 1.9 | - |
| Sex | - | - | - | 40.1% |
| - | Female | 2.28 | -8.68 - 13.24 | - |
| - | Male | 0.28 | -7.11 - 7.66 | - |
| - | Mixed male and female | 0.65 | -8.95 - 10.25 | - |
| Category of disease model induction | - | - | - | 29.3% |
| - | Genetic | -0.31 | -4 - 3.38 | - |
| - | Pharmacological | 1.2 | -0.9 - 3.29 | - |
| Administration route | - | - | - | 29.3% |
| - | Intraperitoneal | -0.31 | -4 - 3.38 | - |
| - | Oral | 1.2 | -0.9 - 3.29 | - |
| Intervention administered | - | - | - | 20.9% |
| - | RO5203648 | -0.31 | -15.03 - 14.41 | - |
| - | RO5256390 | 0.85 | -13.82 - 15.53 | - |
| - | SEP-363856 (Ultaront) | 1.41 | -8.89 - 11.71 | - |
| Drug selectivity | - | - | - | 23.7% |
| - | High | 0.27 | -2.79 - 3.34 | - |
| - | Low | 1.39 | -1.63 - 4.42 | - |
| Drug potency | per log unit | -1.19 | -7.05 - 4.67 | 16.1% |
| Standardised dose | per log unit | -0.21 | -0.42 - 0 | 9% |
| Risk of Bias | - | - | - | 65.6% |
| - | 0 criteria met | 0.43 | -0.97 - 1.84 | - |
| - | 1 criteria met | 2.28 | -0.55 - 5.1 | - |
| Reporting completeness | - | - | - | 0.6% |
| - | 8-11 criteria met | 0.85 | -22.24 - 23.95 | - |
| - | 12-15 criteria met | 0.97 | -15.46 - 17.4 | - |
| - | 16-19 criteria met | 0.65 | -22.08 - 23.39 | - |
2.2.5. Sensitivity Analyses
Imputed 𝞺 values of 0.2 and 0.8
In the previous analyses for the effect of TAAR1 agonists on cognition, we imputed a \(\rho\) value of 0.5. Here, we examine the effect of imputing \(\rho\) values of 0.2 and 0.8.
When the \(\rho\) value is assumed to be 0.2, the TAAR1 interventions had a pooled effect on cognition of SMD = 0.84 (95% CI: -0.27 to 1.95) with a prediction interval of -1.69 to 3.37).
When the \(\rho\) value is assumed to be 0.8, the TAAR1 interventions had a pooled effect on cognition of SMD = 0.65 (95% CI: -0.39 to 1.69) with a prediction interval of -1.5 to 2.81).
For reference the pooled effect size when rho is assumed to be 0.5 is 0.8 (95% CI: -0.3 to 1.9).
Normalised Mean Difference (NMD)
For cognition, 19 out of 19 comparisons, i.e. 100 % of comparisons, had data available for a Sham group and, therefore, to calculate an NMD.
The effect of administering a TAAR1 agonist on cognition in animals using NMD as the effect size is shown in Figure 2.2.5. The pooled estimate for NMD across all individual comparisons is displayed as a diamond shape at the bottom of the plot. Dotted lines indicate the prediction interval of the pooled estimate.
Figure 2.2.5 - Forest plot of TAAR1 agonist vs control on cognitive function using NMD
For TAAR1 Agonist v Control, TAAR1 interventions had a pooled effect on cognition of NMD = 43.1 (95% CI: -38.01 to 124.2) with a prediction interval of -137.84 to 224.03. For reference the pooled effect size for SMD was 0.8 (95% CI: -0.3 to 1.9).
19 experimental comparisons were reported in 5 experiments reported from 4 publications and involving 4 different animal strains.Between-strain variance was 1289.26, between-study variance was 1289.26, and within-study variance (between experiments) was 4.46.
Robust variance estimator (RVE)
Here, we examine the robustness of results when using a sandwich-type estimator to obtain cluster-robust tests and confidence intervals of the model coefficients. The variance-covariance matrix is estimated using the ‘bias-reduced linearization’ for small-sample adjustment and Strain as a clustering variable.
When using the robust variance estimator, TAAR1 interventions had a pooled effect on cognition of SMD = 0.8 (95% CI: -0.58 to 2.18 with a prediction interval of -2.21 to 3.81). For reference the pooled effect size for SMD was 0.8 (95% CI: -0.3 to 1.9), so the using a robust variance estimator does not substantially change the results.
2.2.6 Reporting bias/small-study effects
Because of the relationship between SMD effect sizes and variance inherent in their calculation, where study size is small the standard approach to seeking evidence of small-study effects (regression based tests including Egger’s regression test for multilevel meta-analysis) can lead to over-estimation of small-study effect (see for instance 10.7554/eLife.24260). To address this we used Egger’s regression test for multilevel meta-analysis, with regression of SMD effect size against 1/√N, where N is the total number of animals involved in an experiment.
Egger regression based on 19 effects of TAAR1 Agonist v Control where Locomotor activity was measured showed a coefficient for a small study effect of -68.13 (95% CI: -158.3 to 22.04; p = 0.083).
3 TAAR1 Agonist v known antipsychotic drug
3.1 Outcome 1: Locomotor activity
In TAAR1 Agonist v known antipsychotic drug studies, the effect of administering a TAAR1 agonist on Locomotor activity in animals using SMD as the effect size is shown in Figure 3.1. The pooled estimate for SMD across all individual comparisons is displayed as a diamond shape at the bottom of the plot. Dotted lines indicate the prediction interval of the pooled estimate.
Figure 3.1 - Forest plot of Locomotor activity for TAAR1 Agonist vs known antipychotic drug
For TAAR1 Agonist v known antipsychotic drug comparisons, TAAR1 interventions had a pooled effect on locomotor activity of SMD =-0.622 (95% CI: -1.324 to 0.08, with a prediction interval of-2.272 to 1.029).
21 experimental comparisons were reported in NA experiments reported from 4 publications and involving 7 different animal strains. The between strain variance was 0.37, the between study variance NA and the within-experiment variance NA.
3.2 Outcome 2: Cognitive function
Multilevel analysis is only performed if there are 5 levels or more for at least one of Strain, Study and Experiment, and that is not the case here. We provide a conventional univariate random effects model to illustrate the data
4 Co-treatment with TAAR1 agonist plus known antipsychotic drug v known antipsychotic drug alone
4.1 Outcome 1: Locomotor activity
Multilevel analysis is only performed if there are 5 levels or more for at least one of Strain, Study and Experiment, and that is not the case here. We provide a conventional univariate random effects model to illustrate the data
4.2 Outcome 2: Cognitive function
Multilevel analysis is only performed if there are 5 levels or more for at least one of Strain, Study and Experiment, and that is not the case here. We provide a conventional univariate random effects model to illustrate the data
5 Effect of TAAR1 agonists in TAAR1 receptor knockout animals
5.1 Outcome 1: Locomotor activity
Multilevel analysis is only performed if there are 5 levels or more for at least one of Strain, Study and Experiment, and that is not the case here. We provide a conventional univariate random effects model to illustrate the data
5.2 Outcome 2: Cognitive function
No studies reported cognitive outcomes in TAAR1 knockout animals
6. Proportion of animals not progressing to outcome measurement and adverse effects of treatment
2.58% of 1085 animals in Control cohorts and 3.32% of 1085 animals in Intervention cohorts ‘dropped out’ between allocation to group and outcome measurement. Given that 184 of 213 interventions (86.38%) were administered as a single dose, treatment emergent adverse effects likely to lead to withdrawal of an animal from the study would be unusual, and technical failure or attrition is more likely. This analysis is based on full reporting of animals excluded from analyses, and it may be that group sizes were specified ‘after the event’, or that there was unreported replacement of animals excluded during the experiment, so these data should be interpreted with caution.
7. Summary of the evidence
No studies were of low risk of bias or low reporting bias. Egger Regression suggested that smaller studies report larger effects.
This might occur if smaller studies reporting larger effects were disproportionately represented in those studies for which NMD was not evaluable those which did not report performance in a sham group.
Bias due to indirectness was assessed as follows.
The Initial organism Homological validity
We assessed rodents as having moderate validity as a non primate mammalian species, and all experiments included here were conducted in rodents. Further, 17 experiments reporting locomotor activity and 2 experiments reporting cognition used a genetic manipulation in a gene DAT known to be associated with human disease.
Early environment factors inducing transition from initial organism to vulnerable organism Ontopathogenic validity
Aside from the genetic models above, no models manipulated early environmental factors.
Triggering factors inducing transition from Vulnerable organism to Pathological organism were commonly used e.g. cocaine, amphetamine, PCP but usually only with short term exposures trigerring increased locomotor activity. It was not possible to assess Pathological Organism Mechanistic validity whether the neurobiological or cognitive mechanisms which operate in human disease could be observed in the animal models used or Triggering factors inducing changes in biomarkers Induction validity because these were not assessed.
Therapeutic agents inducing changes in outcome
There was some evidence to support directness in that known antipsychotic drugs had observed efficacy in improving locomotor activity.
Biological markers
There were insufficient studies reporting neuotransmitter levels and no studies reported brain imaging outcomes.
Behavioural manifestations Ethological validity
Most experiments reported locomotor activity, but 19 experiments reported some form of cognitive outcome, and in these there was no significant effect of TAAR1 agonists (0.8, 95% CI: -0.3 to 1.9).
8. Software used
We used R version 4.3.1 (R Core Team 2023) and the following R packages: devtools v. 2.4.5 (Wickham et al. 2022), dosresmeta v. 2.0.1 (Crippa and Orsini 2016), gtools v. 3.9.4 (Bolker, Warnes, and Lumley 2022), Hmisc v. 5.1.1 (Harrell Jr 2023a), kableExtra v. 1.3.9.9001 (Zhu 2023), knitr v. 1.45 (Xie 2014, 2015, 2023), Matrix v. 1.6.1.1 (Bates, Maechler, and Jagan 2023), meta v. 6.5.0 (Balduzzi, Rücker, and Schwarzer 2019), metadat v. 1.2.0 (White et al. 2022), metafor v. 4.4.0 (Viechtbauer 2010), mvmeta v. 1.0.3 (Gasparrini, Armstrong, and Kenward 2012), numDeriv v. 2016.8.1.1 (Gilbert and Varadhan 2019), orchaRd v. 2.0 (Nakagawa et al. 2023), patchwork v. 1.1.3 (Pedersen 2023), PRISMA2020 v. 1.1.1 (Haddaway et al. 2022), rje v. 1.12.1 (Evans 2022), rms v. 6.7.1 (Harrell Jr 2023b), robvis v. 0.3.0.900 (McGuinness and Higgins 2020), tidyverse v. 2.0.0 (Wickham et al. 2019), usethis v. 2.2.2 (Wickham et al. 2023), xtable v. 1.8.4 (Dahl et al. 2019).